The present invention is closely related to an earlier invention entitled "Apparatus and method for handling solids in liquid" as described and claimed in U.S. Pat. No. 4,517,099, issued 14 May 1985 to Raymond A. Breckner, one of the inventors herein. The patent, the disclosure of which is incorporated herein by reference, describes a device having a chamber with inlet and outlet valves, the inlet valve being positioned at an upper portion of the tank so as to be clear of a maximum upper level of liquid within the tank. Mixture is drawn into the tank, when exposed to a low pressure source, through an inlet duct which has an upwardly curved riser means which constrains the mixture to pass initially upwardly, prior to discharging downwardly through the inlet valve. The inlet valve is a hinged plate which is opened when the pressure within the chamber is reduced, and closes when the pressure is increased. The patented device has an outlet valve at a lower portion thereof, which has a hinged plate which is closed by reduced pressure within the chamber, and opened when pressure increases.
The patented device provides a means of ensuring that the inlet valve can close without trapping fish that might otherwise become inadvertently trapped therein. As the chamber fills, the mixture attains a predetermined level at which time the low pressure source is isolated from the chamber The mixture continues to flow through the inlet duct until the pressure differential across the inlet valve is insufficient to maintain the inlet flow, at which time the flow ceases. Any fish that might tend to rest on the inlet valve will fall forwardly through the valve into the chamber, or backwardly into the inlet duct. Prior to raising pressure in the chamber, sufficient time is allowed to permit flow through the inlet valve to cease, and to permit any solids that might collect adjacent the valve to fall from the valve, thus permitting the inlet valve to close cleanly. The above patent thus discloses an arbitrary time delay to ensure that flow in the inlet duct has ceased prior to closure of the inlet valve. However, it does not provide any means of directly measuring when flow through the inlet duct has actually ceased, and thus, to accommodate many variables in different installations, it is usual to provide a conservatively excessive time for flow through the inlet duct to cease, prior to raising pressure in the chamber.
While providing an excessive time for flow through the inlet duct to cease ensures that the inlet valve can close without trapping solids upon raising of the pressure, in many situations excessive time is wasted when there is no flow in the duct and the pressure in the chamber is not rising.
Pumps for conveying fish immersed in water, for example from fish holds onto a dockside, or within a fish processing plant, have been used for many years. It is well known that fish debris, e.g. fish scales, fins, roe, etc., from damaged fish is highly adhesive or tenacious and is prone to sticking to any mechanism that is exposed to the debris, and in a short while the mechanism can become seized and no longer functions. Clearing fish debris from such mechanism can be very time consuming, as sometimes considerable mechanical force is required to remove the contaminants. Consequently, any attempt to directly detect or sense flow of a liquid/fish mixture in a duct is susceptible to seizure if the apparatus is not used for some time. Because fish handling equipment tends to be used only during and shortly after the fishing season, such equipment tends to be left for long periods of time without being used, and even after thorough cleaning, is prone to seizure due to tenacious characteristic of fish debris.